The present invention relates to a magnetoresistive head and manufacturing method thereof, and a read write separation type head in which a magnetoresistive head is installed, and in particular to a structure of the magnetic domain control layer of a magnetoresistive head.
A magnetic head used in a magnetic storage device (hard disk drive) has a recording head unit which records signal information in a magnetic recording medium (hard disk) as magnetization information, and a read head which transforms the magnetization information recorded in the magnetic recording medium into signal information and reads it. A magnetic recording medium is required to increase a recording information capacity, improve the transformation rate, and decrease the error rate, and so the magnetic recording density is being improved from year to year. The improvement of the magnetic recording density lowers the volume of unit magnetic information recorded in the magnetic recording medium, which in turn decreases the amount of magnetization of magnetic information per unit; as a result, the read head needs higher sensitivity than in the past. At the same time, generally as the recording density is improved, noises of the magnetic recording medium and the read head and also instability of read signals are increased. Accordingly, today, the read head needs low noise and signal stabilizing technology more than ever.
The read head includes a magnetoresistive stack due to requirements of high sensitivity in high recording density and reads signals through the use of the magnetoresistive effect, and is thus called a magnetoresistive head. The stack structures of the magnetoresistive heads nave various types and are classified into a GMR head, a CPP-GMR head, a TMR head, and other heads on the basis of their using magnetoresistive principles; they are being studied based on the classifications. The heads respectively make use of the GMR (Giant Magnetoresistive effect), the CPP-GMR (Current Perpendicular Plane GMR effect), and the TMR (Tunneling Magnetoresistive effect) and output as voltage changes from input magnetic fields which enter the read head from the magnetic recording medium.
Each of these magnetoresistive heads includes a magnetic sensor unit having a pinned layer in which the magnetization direction is fixed, a nonmagnetic layer formed by making contact with the pinned layer, and a free layer formed by coming in contact with the nonmagnetic layer, and the end of the magnetic sensor unit. The end has a magnetic domain control layer which controls the magnetization direction of the free layer and an electrode layer which flows a current into the magnetic sensor unit. The magnetic sensor has a ferromagnetic shield layer disposed in the upper and lower portions thereof. The free layer utilizes the input magnetic field from magnetic information recorded in the recording medium for the change of the magnetization direction. When the magnetization direction of the free layer is changed, the difference between the free layer and the pinned layer directions alters the resistance of the magnetic sensor unit. The hard disk drive has a construction in which the resistance change is read in the form of an electrical signal. This requires causing a current to flow for the reading of the resistance change in the magnetic sensor unit, so that the magnetic sensor end is provided with an electrode layer together with a magnetic domain control layer.
The magnetic domain control layer is disposed in the free layer end and has a structure in which it applies a bias magnetic field to the free layer. This is because although the magnetization direction of the free layer is constructed so as to be changed sensitively with a very weak recording magnetic field of information recorded in the recording medium, bias magnetic field application is required for the guarantee of reproducibility and stability of the magnetic conditions when the free layer is changed between the initial magnetic conditions and the magnetic conditions receiving recording magnetic field input. In other words, the free layer, whose magnetization is rotated by receiving input magnetic field from the magnetic recording medium, is not made a single domain. If the free layer has a magnetic domain, the magnetic domain wall is moved, which leads to causes of a variety of noises such as the Barkhausen noise, and which loses the reproducibility of the magnetic conditions between the initial magnetic conditions and the magnetic conditions receiving recording magnetic field input resulting in phenomena such as output variation, thereby being incapable of signal reproduction. In order that the magnetic domain of the free layer is made a single magnetic domain, the magnetic domain control layer has a structure in which a bias magnetic field is applied to the free layer.
The magnetic sensor unit is placed within a magnetic shielding layer so as to sensitively react only with a very weak recording magnetic field recorded in the recording medium, not to be affected by other external magnetic fields, and to efficiently guide the medium recording magnetic field to the free layer of the magnetic sensor unit. The magnetic sensor unit and the magnetic domain control layer and the electrode layer disposed outside both ends thereof need to be electrically shielded from the magnetic shield layer disposed on the upper and lower portions thereof, and thus the space between the layers are comprised of an electrically insulating material.
Requirements of high sensitivity concerning the read head in high recording density, reduction of noise, and signal stabilization stimulates not only studies on improvement of the magnetic sensor unit, but also various investigations of magnetic domain control layers, electrode layers and shielding layers. For instance, Patent Document 1 (Japanese Patent Laid-Open No. 2004-103120) proposes an approach in which the adoption of a differential bias method for the magnetic domain control layer enables the application of an appropriate bias magnetic field by means of the free layer. The differential bias construction, which uses two magnetic domain control layers, is a system which involves directing the magnetization of the free layer to a given direction in a first magnetic domain control layer, providing the second magnetic domain control layer slightly separated from the first magnetic domain control layer, and directing the magnetization of the second magnetic domain control layer to the opposite direction of the first magnetic domain control layer for the application of a bias magnetic field. This approach is considered to maintain high a bias magnetic field of the end of a magnetoresistive stack, to be capable of achieving a bias magnetic field distribution which rapidly decreases toward the center of the magnetoresistive stack, and to be effective in high output as well as to readily attain stabilization of a read output because of being capable of causing the free layer to be efficiently made single magnetic domain.
Also, Patent Document 2 (Japanese Patent Laid-Open No. 2004-119534) discloses an intermediate stop hard bias construction and manufacturing process thereof for improvement of characteristics of the magnetic domain control layer in the free layer end and also for an appropriate application of the bias magnetic field of the magnetic domain control layer to the free layer. In this intermediate stop hard bias construction, setting equal the position of the heights of the free layer and the magnetic domain control layer makes small the layer thickness of the magnetic domain control layer, thereby localizing a bias magnetic field from the magnetic domain control layer and minimizing, in an attempt to simultaneously achieve the high output and stabilization of a read head signal. Moreover, in the description of Patent Document 2, this construction makes use of a Cr underlying layer when a Co alloy layer is used as an improved material of the magnetic domain control layer, and utilizes a metal amorphous seed layer to further regulate the crystalline structure of the Cr underlying layer. Furthermore, in the description of Patent Document 2, the thinner the layer of the metal amorphous seed layer and the Cr underlying layer, the better it is, in the range of the coercivity and the squareness of the Co alloy layer being maintained high, for improvement of the hard bias characteristics. Further, the higher the saturation magnetic flux density and the residual magnetic flux density of the Co alloy layer and also the thinner the layer of the Co alloy layer, the better it is.
Patent Document 1 above proposes a differential bias method which makes the magnetic layer construction two layers as an improved material of the magnetic domain control layer; however, this method is difficult to adopt since no space is present for the formation of many thin layers in the end of the magnetoresistive stack (GMR layer).
In Patent Document 2, an attempt is made to improve the characteristics of the magnetic domain control layer for the improvement of the characteristics of the read head, but making thin the layer of the magnetic domain control layer is limited, having shown that improvement of the read head characteristics exhibits limitations.
Limitations of the Composition Characteristics of the Co Alloy Magnetic Layer
As described in Patent Document 2, the magnetic domain control layer includes a Co alloy magnetic layer and a Cr underlying layer, and a metal amorphous seed layer which controls its crystalline structure on account of the need for a high coercivity and a high magnetic flux density. The examples set forth therein include cases which make use of a CoCrPt alloy magnetic layer, a Cr underlying layer and an NiTa alloy amorphous seed layer. Also, at the same time, the document describes an approach which involves altering the content of the Cr and Pt additional elements of the CoCrPt magnetic layer for the purpose of attaining a high magnetic flux density, thereby having realized the magnetic properties of a high magnetic flux density to have improved magnetic domain controllability. The Cr composition and the Pt composition are added for the purposes of the improvement of coercivity while maintaining a high magnetic flux density and also the improvement of corrosion resistance and the adjustment of the saturation magnetic flux density. The characteristics of the magnetic domain control layer which is excellent include Co alloy compositions which show a high saturation magnetic flux density Bs, a high residual magnetic flux density Br and a high coercivity Hc. The free layer comprises a CoFe alloy and an NiFe alloy film, and the saturation magnetic flux density Bs is from 1T inclusive to 2T. Therefore the residual magnetic flux density of the magnetic domain control layer preferably seems to be 1T or higher. In addition, the coercivity Hc of the magnetic domain control layer is preferably 80 kA/m or more and its low limit is thought to be from 64 kA/m to 48 kA/m.
However, limitations have come to be apparent for the formation of a magnetic domain control layer which simultaneously offers a high saturation magnetic flux density Bs, a high residual magnetic flux density Br and a high coercivity Hc by the preparation of a Co alloy composition. More specifically, the amounts of the Cr composition and the Pt composition must be decreased in order to obtain a high saturation magnetic flux density Bs and a high residual magnetic flux density Br, but if the compositions are decreased, the coercivity Hc is lowered. The coercivity Hc of a CoCrPt film greatly varies depending on the Pt composition, in particular the coercivity Hc is clearly rapidly lowered when the Pt content is 10 at % or less. In other words, although the adjustment of the additional element composition of a Co alloy magnetic layer readily forms a magnetic domain control layer of a high coercivity Hc, a low saturation magnetic flux density Bs and a low residual magnetic flux density Br, and a magnetic domain control layer of a high coercivity Hc, a low saturation magnetic flux density Bs and a low residual magnetic flux density Br, it has apparently become difficult to obtain a magnetic domain control layer of a high coercivity (Hc≧120 kA/m), a high saturation magnetic flux density Bs and a high residual magnetic flux density Br(Br≧1T).
Problems of Degradation of Magnetic Properties During Making Co Alloy Magnetic Layer Thin
Additionally, Patent Document 2 points out that the magnetic domain controllability of a free layer is lowered because of the deterioration of magnetic properties of a magnetic domain control layer formed on the GMR layer end, and describes an approach which avoids a decrease in its magnetic properties. Moreover, the document also sets forth that in the proximity of the GMR layer end, the magnetic domain control layer is thin as compared with an even portion on account of the manufacturing method thereof.
In general, it is known that the magnetic properties of a CoCrPt/Cr base alloy magnetic layer decrease as the thickness of the magnetic layer is small. FIG. 9 indicates the results in the case where the dependency of the magnetic properties on the layer thicknesses of magnetic layers was investigated when the Cr content and the Pt content of a CoCrPt alloy magnetic layer are changed. The Cr content and the Pt content are selected so that the magnetic flux density Bs and the coercivity are as high as possible. As the thickness of the CoCrPt magnetic layer is decreased, the coercivity Hc, the saturation magnetic flux density Bs, the residual magnetic flux density Br and the squareness S decrease. In particular, when the layer thicknesses are 3 nm or less, they rapidly decrease; for the layer thicknesses in the proximity of 1 nm, the characteristics of the hard magnetism disappeared. These characteristic degradation phenomena depend on thermal magnetic phenomena and the deterioration of the crystalline structure when Co alloy layers are made thin, so the phenomena are unavoidable even though the manufacturing process or the Co alloy composition is improved.
The problem of a deterioration in magnetic properties when the magnetic layer is made thin is estimated to also occur in a magnetic domain control layer in the proximity of the end of a GMR layer. More specifically, when the thickness of a magnetic domain control layer close to the end of a GMR layer is 3 nm or less, the magnetic properties of the magnetic domain control layer greatly lower, which is estimated to lead to a large decrease in magnetic domain control magnetic field to the free layer. In addition, when the thickness of the magnetic domain control layer is small, magnetic anisotropy energy KuV (Ku: magnetic anisotropy energy per volume, V: volume of crystal grain) greatly decreases, magnetic dispersion in the end of the magnetic domain control layer is likely to increase. As such, disturbances such as a temperature increase in magnetoresistive head actuation and an input magnetic field from the magnetic recording medium lower the magnetic domain control bias magnetic field to the free layer. This leads to malfunctions such as occurrences of Barkhausen noises and waveform variation, or enlargement of asymmetry of input.
Problems of Magnetic Domain Control Layer Intermediate Stop Structure
Patent Document 2 is directed to an invention which focuses attention on the improvement of the characteristics of the magnetic domain control layer. As a result, the document describes the improvement of the magnetic domain control characteristics by setting equal the position of the heights in the forming of the magnetic domain control layer and the free layer of the GMR layer. Also, the read head characteristics include as important characteristics the read head resistance characteristics as well as the magnetic domain control characteristics; the lower the read head resistance characteristics, the better it is. More specifically, sense current flows into the GMR sensor unit through the electrode layer formed on the upper portion of the magnetic domain control layer and then via the electrode layer and the bound portion between the magnetic domain control layer and the GMR layer end. If the electrode layer resistance is high or the electrode layer and the bound portion between the magnetic domain control layer and the GMR layer end are high, a temperature rise in the proximity of the GMR layer occurs due to the sense current, which leads to the generation of deterioration of the GMR characteristics and also the cause of noises. When the read track width is small, this detriment is increased.
On the other hand, when the read track width is small, the electrode layer formed in the GMR layer end renders the shape of the upper magnetic seed layer concave, and increases the average interval between the upper magnetic shield layer and the top surface of the GMR layer. It is well known that an increase in the average interval between the upper magnetic shield layer and the top surface of the GMR layer leads to a decrease in the read characteristics during magnetic read/write processing. Accordingly, the upper magnetic seed layer is preferably formed as even as possible and the upper magnetic shield layer must be avoided to be concave due to a thick electrode layer. Namely, the adoption of an intermediate stop structure limits the space in which the electrode layer is formed, being incapable of forming the electrode layer thick. As a result, apparently, a simple increase in the electrode layer thickness cannot make the read head resistance low. Making the magnetic domain control layer thin enables the electrode layer to be thick by a layer thickness made thin of the magnetic domain control layer, thereby readily obtaining low resistance characteristics. Hence, even when the magnetic layer thickness is small, there are required a magnetic domain control layer having good magnetic properties and high stability, and a forming method thereof.
The adoption of the intermediate stop structure described in Patent Document 2 attains great improvement in the magnetic domain control layer characteristics, but the structure has a disadvantage in that resistance is low in the electrode layer and the bound portion between the magnetic domain control layer and the GMR layer end.